In a paper-making process, chemical pulping yields, as a by-product, black liquor which contains almost all of the inorganic cooking chemicals along with the lignin and other organic matter separated from the wood during pulping in a digester. The black liquor is burned in a boiler. Two functions of the boiler are to recover the inorganic cooking chemicals used in the pulping process and to make use of the chemical energy in the organic portion of the black liquor to generate steam for a paper mill. As used herein, the term boiler can include either a bottom supported boiler or a top supported boiler that, as described below, burns a fuel which fouls heat transfer surfaces.
A Kraft boiler, for example, can include superheaters in a furnace that extract heat by radiation and convection from the furnace gases. Saturated steam enters the superheater section and superheated steam exits at a controlled temperature. The superheaters comprise an array of platens that are constructed of tubes for conducting and transferring heat. Superheater heat transfer surfaces are continually being fouled by ash that is being carried out of the furnace chamber. The amount of black liquor that can be burned in a Kraft boiler is often limited by the rate and extent of fouling on the surfaces of the superheater section. The fouling, including ash deposited on the superheater surfaces, reduces the heat absorbed from the liquor combustion, which can result in reduced exit steam temperatures from the superheaters.
Boiler shutdown for cleaning is sometimes required. For example, Kraft boilers are particularly prone to the problem of superheater fouling. One conventional method of removing ash deposits from the superheaters in Kraft boilers includes sootblowing. Sootblowing is a process that includes blowing deposited ashes off the superheaters (or other heat transfer surface that is fouled with ash deposits) with a blast of steam from nozzles of a sootblower. A sootblower has a lance for conducting the steam to the nozzles at a distal end of the lance. Sootblowing can be performed essentially continuously during normal boiler operation, with different sootblowers turned on at different times. Sootblowing is usually carried out using steam. The sootblowing procedure consumes a large amount of thermal energy produced by the boilers.
Conventional methods of determining the amount of deposit on superheater sections of recovery boilers are typically based on indirect measurements, such as the temperature increase of exhaust gas exiting the boiler, the temperature decrease of steam, heat transfer, enthalpy, or the pressure drop increase over the gas side (combustion section as opposed to the water/steam side) of the boiler. However, such techniques are unable to determine, or at least address, the location of the ash deposits on the heat exchangers of the recovery boilers. Accordingly, such techniques offer little guidance in providing a reliable and efficient sootblowing strategy that reduces steam consumption for energy savings or to improve heat transfer surface effectiveness.